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He said: “Many non-scientific people seem to think we are generating new species so they can expand and eventually they may take over. But we are just making an organism with both human and sheep cells. It’s nothing like a pig with a human face or human brain.” His latest project was outlined at the annual meeting of the American Association for the Advancement of Science.

Dr Nakauchi told how he has injected pluripotent stem cells — which can become any organ — from a human into the embryos of sheep genetically modified not to produce a pancreas.

It is hoped the human stem cells will fill the void as the embryos develop.

If successful, it could lead to other large animals being able to grow organs such as the heart, liver and lungs.

Dr Nakauchi revealed stage one of his study had developed embryos with 0.1% human cells to 21 days of gestation. He said they had also found a way for these cells to multiply to form a certain organ.

Stage two will see researchers applying for approval to extend this to 70 days and at the same time “knock out” the sheep’s ability to grow its own pancreas. Prof Nakauchi added: “It could take five or 10 years, but I think eventually we will be able to do this.”

Geneticist Prof Martin Bobrow, of Cambridge University, said “1% of human cells could be enough” to develop the pancreas.